The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with an improved process for preparing polylaurolactam coating compositions for coating metal objects at high temperatures where the polylaurolactam powder is obtained from polylaurolactam granulates which granulates in turn are obtained by polymerizing laurolactam in the presence of 2 to 10% by wt. of water, 0.3 to 0.7% by wt. of phosphoric acid and at temperatures between 265.degree. and 300.degree. C. in the absence of any chain stabilizers and under internal pressure of about 14 to 25 bars.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. Nos. 2,698,966; 2,742,440; 2,975,128; 2,993,879; 3,203,822; 3,299,009; 3,321,447; 3,476,711; 3,564,599; 3,900,607; 3,927,141; 3,966,838 and 4,143,025 and the Kirk-Othmer "Encyclopedia of Chemical Technology" 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92-polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein.
U.S. Pat. No. 2,698,966 discloses physical mixtures of different types of nylon powders produced by dissolving the nylon in organic solvents and precipitating the powdered particles. In Example 14 of U.S. Pat. No. 2,742,440, the solution of epsilon caprolactam in alcohol-water solution and its precipitation as a powder is disclosed. The dispersion of Teflon powder in a solution of epsilon caprolactam and the coprecipitation of the powders is disclosed in U.S. Pat. No. 2,975,128.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822. U.S. Pat. No. 3,410,832 discloses the preparation of polymers and copolymers of lauryllactam (laurolactam).
It is known to use polyamide based powder coating compositions in the preparation of coatings similar to lacquer on metals. The coatings are applied by a vitrification coating process. The term includes the fluidized bed coating method, the flame spraying method and the electrostatic coating method.
An especially advantageous process is described in U.S. Pat. No. 4,143,025. This procedure however, can stand improvement. The procedure of U.S. Pat. No. 4,143,025 requires observing two different operational sequences for (A) non-pigmented and (B) pigmented coating powders. Thus, the non-pigmented (A) so-called natural powders which are obtained by grinding, must be post-condensed in the solid state, whereby the space-time yield is lowered and higher energy consumption is incurred.
The pigmented powders (B) on the other hand, require a wholly different preparation, namely, precipitation from solvents. Accordingly, the manufacture of the different powders requires two different production lines, whereby a substantially higher cost is experienced, and this also entails the limitation of dealing with solvents.
Lastly, the powders obtained by the described state of the aret are not yet sufficiently satisfactory as regards quality. Both the so-called natural powders and the pigmented powders evidence a strong drop in elasticity, especially following the so-called "dry-blend pigmentation" and when substantially thick metal parts are coated. Furthermore, the powders evidence low yellowing resistance (overbaking stability). This drawback shows itself in that differences in color occur when metal parts of different thicknesses are coated. This varyingly pronounced yellowing is found both in the natural powders and in those obtained from "dry-blend pigmentation".
By "dry-blend" process is meant the application of shear and heat to a blend of polymer plus additives, resulting in an absorption of the additives into the softened polymer. Especially high speed mixers are used. By the process a dry free-flowing blend (powder) is obtained. No solvents are used. (Developments in PVC Technology, Chapter 6, pp. 90 to 106).